1. Field of the Invention
The present invention relates to the control circuit of a flyback-type switching power supply system and the flyback-type switching power supply system and more particularly to the control circuit of a switching power supply system and the switching power supply system which are formed so as not to vary the characteristics of a power supply operation frequency to a load factor even though the voltage of an AC input differs.
2. Background Art
A switching power supply system is able to convert a commercial AC voltage to a DC voltage with an arbitrary voltage to output the DC voltage. The system has a low parts count and is adaptable to a wide range of input voltages. As a system in which an output voltage is isolated from the voltage of a commercial power supply, the system of a flyback-type is known. The flyback-type switching power supply system is provided with an input circuit that converts an AC input voltage to a DC voltage, a transformer, a switching device, an output circuit that converts a secondary side voltage of the transformer to a DC voltage (an output voltage) and a control circuit that carries out on-off control of the switching device. The control circuit controls the turning-on width of the switching device connected in series to the primary winding of the transformer on the basis of the output voltage of the output circuit to thereby carry out control so that the output voltage becomes constant.
The control circuit also detects the load condition of the switching power supply system. In the case when the system is under a high-load condition, the control circuit controls the turning-on and -off of the switching device by a fixed high frequency wave. In the case when the system becomes under a light-load condition, however, the control circuit generally carries out a control in which the switching frequency of the switching device is decreased to decrease a switching loss (see JP-A-11-155281 and JP-A-2004-40856 (Paragraph [0025] and FIG. 11), for example).
In the invention described in JP-A-11-155281, although the invention is not the invention of a flyback-type switching power supply system, an output voltage is detected for maintaining a high switching frequency when the output voltage is decreased under a high-load condition. Meanwhile, an increase in the output voltage due to a light-load condition decreases the switching frequency with the increase in the output voltage so as to thereby improve the efficiency of the light-load.
Similarly, in the invention described in JP-A-2004-40856, the voltage of an output circuit is also detected and when the value of a fed back voltage is equal to or more than a specified value, a capacitor that determines the oscillation period of an oscillator is charged by a constant bias current to make the oscillator oscillate at a certain constant oscillation frequency. In a region in which the value of the fed back voltage is equal to or less than a specified value, the value of a bias current to be charged in the capacitor decreases to lengthen a charging time until the voltage across the capacitor exceeds the threshold level, by which the switching frequency is going to decrease.
In this way, a switching power supply system has a decreasing characteristic of an oscillation frequency to a load factor. When making the specification of such a switching power supply system in compliance with a worldwide specification, commercial power supplies of a 100V system and a 200V system are to be connected to the AC input of the system. At this time, the switching power supply system is designed so as to be in compliance with a wide range of AC input voltages with components and various kinds of set values designed to be identical.
FIG. 10 is a characteristic diagram showing power supply operation frequency decreasing characteristics relative to load factors for different AC input voltages.
The characteristic diagram shown in FIG. 10 has the horizontal axis representing load factor and the vertical axis representing power supply operation frequency to show power supply operation frequency (switching frequency) decreasing characteristics relative to the load factor in the cases in which the values of the AC input voltages Vin are for the 100V system (AC115V) and for the 200V system (AC230V).
According to the characteristic diagram, in each of the cases in which the values of the AC input voltages Vin are for the 100V system and for the 200V system, the characteristic is presented so that under a condition with a load factor being higher than a certain one, the power supply operation frequency is constant and under a condition with a load factor being lower than a certain one, the power supply operation frequency decreases with a decrease in the load factor. However, while the load factor at which the power supply operation frequency initiates to decrease is 47% in the 100V system, such load factor becomes 54% in the 200V system. This is because, under conditions of the same power supply operation frequency and load factor, a higher AC input voltage brings a higher increasing rate of the energy stored in a transformer, by the increased amount of which energy the amount of feedback increases so as to narrow the turning-on period of the switching device to result in the initiation of a decrease in the power supply operation frequency at a higher load factor.
In a switching power supply system, a power supply operation frequency as a switching frequency decreases as a load becomes lower to finally enter the audible region. With a switching frequency in the audible region, the core of a transformer vibrates at the frequency in the audible region. This causes a sounding phenomenon that produces abnormal sound from a transformer. Therefore, when the power supply operation frequency decreases to the audible region, a vibration isolating measure of some kind becomes necessary.
However, in the case where an AC input voltage is the voltage of the 100V system and in the case where an input AC voltage is the voltage of the 200V system, their respective power supply operation frequency decreasing characteristics show different ones in which their respective timings differ at each of which the power supply operation frequency enters the audible region. This caused a problem in that the switching power supply system, although it is in compliance with a world wide specification in the configuration, requires individual adjustments for vibration isolation depending on the magnitude of the power supply voltage to be used with respect to the vibration isolating measure.
The invention was made in view of such point with an object of providing a control circuit of a switching power supply system and the switching power supply system which require no vibration isolating measures depending on the magnitude of an AC input voltage.